1. Technical Field
The present invention relates to a dye sensitizing type photoelectric conversion element and a solar cell that is constituted with the photoelectric conversion element.
2. Description of Related Arts
In recent years, solar light energy has drawn attention as an energy source because of environmental problems, and a method of converting light and heat by solar light energy into electric energy as a more usable energy form has been put into practical use. In particular, a method of converting the solar light energy into electric energy is typical, and a photoelectric conversion element is used in this method. For the photoelectric conversion element, a photoelectric conversion element using inorganic materials such as single crystal silicon, polycrystal silicon, amorphous silicon, cadmium telluride and copper indium selenide is widely used for, so-called, a solar cell. However, a solar cell obtained by using a photoelectric conversion element using these inorganic materials has a multiple layered pn junction structure in which a silicon, or the like used as a material is required to be a high purity product obtained by undergoing a high purification process, and thus, there has been problems such that production steps are complicated and the number of processes is large, and a production cost is high.
On the other hand, researches of a photoelectric conversion element using an organic material as a simpler element have progressed. For example, pn junction type organic photoelectric conversion element obtained by connecting a perylenetetracarboxylic acid derivative as an n-type organic dye and copper phthalocyanine as a p-type organic dye has been reported. In order to improve a short exciter diffusion length and a thin space charge layer, which are considered to be defective points in an organic photoelectric conversion element, an attempt to simply increase an area of pn junction portion to which organic thin films are laminated to ensure a sufficient number of organic dyes involved in charge separation has been achieving results.
Further, there has been a technique in which a pn conjunction portion is significantly increased by complexing an n-type electron conductive organic material and a p-type hole conductive polymer in a film to carry out charge separation in the entire film. Heeger, et al. suggested a photoelectric conversion element obtained by mixing a conjugate polymer as a p-type conductive polymer and fullerene as an electron conductive material in 1995.
These photoelectric conversion elements has gradually improved characteristics thereof but has not achieved operating stably with keeping high conversion efficiency.
However, in 1991, Gratzel performed huge and detailed experiments of a sensitizing photoelectric current of a dye adsorbed on titanium oxide, to succeed in preparation of a photoelectric conversion element that operates stably and has high conversion efficiency by forming titanium oxide into being porous, and sufficiently securing the charge separation area (the number of molecules contributing to charge separation) (for example, see B. O'Regan and M. Gratzel: Nature, 353, 737 (1991)).
In this photoelectric conversion element, a cycle in which a dye adsorbed on the surface of porous titanium oxide is photoexcited, electrons are injected into titanium oxide from the dye to form a dye cation, and the dye receives electrons through a hole transport layer from a counter electrode is repeated. For the hole transport layer, an electrolytic solution obtained by dissolving electrolyte containing iodine into an organic solvent has been used. This photoelectric conversion element has excellent reproducibility in cooperation with stability of titanium oxide, bases of research and development are largely extended. This photoelectric conversion element is also called a dye sensitizing type solar cell, which receives high expectation and attention. In this method, an inexpensive metal compound semiconductor such as titanium oxide is not required to be purified to high purity, an inexpensive one can be used as the semiconductor, further, available light ranges over the wide visible light region, and this method has an advantage such that the solar light having many visible light components can be effectively converted into electricity.
However, since a ruthenium complex having restriction as the resource is used for a photoelectric conversion layer, there are problems such that an expensive ruthenium complex is required to be used, stability with time is not sufficient, and the like. In addition, as a further problem, a dye sensitizing type solar cell operates using an electrolytic solution as described above and thus had a problem of requiring another mechanism to prevent retention, and effluence and dissipation of the electrolytic solution and iodine.
Development of an all solid-state dye sensitizing type solar cell that avoids such problems of elution of an electrolytic solution has been also progressing. For example, the all solid-state dye sensitizing type solar cell using an amorphous organic hole transport agent and the all solid-state dye sensitizing type solar cell using copper iodide as a hole transport agent have been known. Since conductivity of the hole transport agents is low, however, such cells have not yet attained sufficient photoelectric conversion efficiency.
Typical examples of a hole transport agent having comparatively high conductivity include polythiophene-based materials, and an all solid-state dye sensitizing type solar cell using polyethylene dioxythiophene (PEDOT) as a hole transport agent has been reported (for example, see JP-A-2003-317814 and J. Xia, N. Masaki, M. Lira-Cantu, Y. Kim, K. Jiang and S. Yanagida: Journal of the American Chemical Society, 130, 1258 (2008)). However, since PEDOT has absorption in the visible light region (400 to 700 nm), loss for light absorption of a dye is generated, and thus photoelectric conversion efficiency has not been sufficient yet.
On the other hand, it has been reported that when the PEDOT substituent described in L. Groenendaal, G. Zotti and F. Joans, Synthetic Metals, 118, 105 (2001) is used, absorbance in the visible light region is reduced while maintaining a conductivity, and an all solid-state dye sensitizing type solar cell using the PEDOT substituent as a hole transport agent has also been reported (for example, see JP-A No. 2000-106223).
On the other hand, development of an all solid-state dye sensitizing type solar cell using an organic dye in place of the ruthenium complex has also been progressing in order to reduce the cost of the solar cell. For example, the all solid-state dye sensitizing type solar cell using an organic methine dye having a thiophene backbone (WO 04/082061 A) has been reported and the all solid-state dye sensitizing type solar cell using an organic dye having a polythiophene backbone with a repetition number of 5 or more (JP-A-2005-135656) has been reported.